Physiological Value and Critical Role of the Bicarbonate-Carbonic Acid (CO2-HCO3-) Buffer
At 6.1 the pKA of the CO2-HCO3-
buffer is not close to the desired plasma pH of 7.4. The concentration
ratio of bicarbonate to carbon dioxide is 20:1. The unique physiological
advantage of this buffer system is that the acid form (CO2)
and salt form (HCO3-) can be regulated independently.
Excretion or retention of CO2 is controlled by the lung and
reabsorption and regeneration of HCO3- is controlled
by the kidney.
A second advantage is that there is a readily available supply of
CO2 from cellular metabolism.
A third factor is the location of this system in the ECF. As the lung and kidney control the ECF pH, H+ (as such or in the form of CO2) moves along its concentration gradients from one compartment to the other. As the renal and pulmonary mechanisms operate to return ECF pH toward normal, H+ buffered intracellularly at the onset of an acid load will move out of the cells into the ECF where it reacts with the CO2 HCO3- buffer system. This cycling of H+ from ICF to ECF Buffer will continue until the acid load has been eliminated and the ECF HCO3- concentration restored to normal. While the movement of CO2 across cell membranes is rapid the movement of H+ and HCO3- is much slower and ICF buffering, which accounts for about half the body buffer, is slow requiring hours to take effect in comparison to the ECF Buffers which react to changes in pH very quickly. The exception to this is the RBC membrane which is highly permeable to HCO3- and Cl- as well as to CO2. This makes the buffering capacity of intracellular hemoglobin as efficient as the CO2 - HCO3- system in buffering pH changes in the ECF.